In the treatment of nature's water for human consumption, man has developed several chemical methods along the way to make safe to drink and enhance its purity. Initially, disinfection was an early issue. Halogens such as (bromine, chlorine, iodine, et. al.) proved useful to disinfect pathogens found in water. Chlorine early took control of the market due to cost and availability. Other chemicals have proved useful for disinfection such as hydrogen peroxides, (ClO2), permanganates, chloroamines, bleach, et. al.), and research continues the study of pathogen removal. However, these disinfectants also are classified as chemical oxidizers which may need consideration in chemical processes in aqueous systems (water treatment).
Dirt or turbidity removal from surface water became an issue and AlSO4 was an early coagulant useful to remove (dropout) particulate. Other chemicals and chemical blends have since been found useful to remove dirt and research continues to develop new and improved chemical products for this application.
Similarly, corrosion in iron piping for transporting drinking water to communities became a concern. An early effort at corrosion control used chemicals and the aqueous property of pH to effect internal coating of the pipes with calcium carbonates. The Langelier Saturation Index is a calculated number used to predict the calcium carbonate stability of water. It indicates whether the water will precipitate, dissolve, or be in equilibrium with calcium carbonate. Langelier developed a method for predicting the pH at which water is saturated in calcium carbonate. The LSI is expressed as the difference between the actual system pH and the saturation pH. For LSI>0, water is super saturated and tends to precipitate a scale layer of CaCO3. For LSI=0, water is saturated (in equilibrium) with CaCO3. A scale layer of CaCO3 is neither precipitated nor dissolved. For LSI<0, water is under saturated and tends to dissolve solid CaCO3. If the actual pH of the water is below the calculated saturation pH, the LSI is negative and the water has a very limited scaling potential. If the actual pH exceeds the saturation pH, the LSI is positive, and being supersaturated with CaCO3, the water has a tendency to form scale. At increasing positive index values, the scaling potential increases. It is also worth noting that the LSI is temperature sensitive. The LSI becomes more positive as the water temperature increases. This increase in temperature can cause scaling, especially in cases such as hot water heaters. Conversely, systems that reduce water temperature will have less scaling. However, the use of the Langlier Index did not prove applicable in some waters and was difficult to use in many other waters. Also, in many surface water treatment systems the precipitation of calcium accumulates to the point of restricting water flow rate.
The water often contains dissolved salts of barium, calcium, magnesium, etc. which exist in such waters in form of soluble salts, usually sulphates, bicarbonate, or chlorides with the soluble salts being ionized so that the waters contain a relatively large concentration of free calcium and/or magnesium ions which can lead to scale and sludge deposits. Other metal ions such as iron or aluminum may be present as contaminants. Scale can cause rapid localized corrosion and subsequent penetration of metallic surfaces through the formation of differential oxygen concentration cells and is often referred to a as under deposit corrosion. Upon application of water softening compounds such as sodium carbonate, trisodium phosphate, sodium phosphate and sodium silicate the calcium and magnesium ions in the water are converted in part into insoluble salts which are precipitated and removable; however, the free sodium ions remain and result in water having a high alkaline content which is undesirable due to its damage to skin or fabrics. The alkaline cations such as calcium, magnesium, iron, copper, aluminum and silica ions form scale deposits which crystallize directly on inner metal surface of water conduits and containers, and sludge deposits of salts that have precipitated therefrom which settle at low flow points.
Inorganic phosphates such as (tripolyphosphoric acid, pyrophosphoric acid, hexametaphosphoric acid), and organic phosphoric esters acids such as (alkyl phosphate and alkyl phosphite) were introduced to reduce calcium carbonate scaling in water lines by eliminating calcium carbonate scale formation. However, as set forth in U.S. Patent publication 20020017494 by Haase, inorganic polyphosphoric acids, phosphonic acids and organic phosphoric esters used in low concentration can adversely act to enhance corrosion and when added in high concentration can lead to the formation of scale in that the inorganic polyphosphoric acids are hydrolyzed in water to produce orthophosphoric acid ions which act upon polyvalent metal ions such as the calcium ions to form insoluble precipitates. The phosphonic acids and organic phosphoric acid esters are hydrolyzed in water to form insoluble precipitates.